Myocarditis is a disease caused by infection, allergy, poisoning or the like which will lead to inflammatory myocardial disorders and most of myocarditis is believed to be viral. As causative viruses, a number of enteroviruses are considered. In particular, Coxsackie B virus is believed to cause viral myocarditis most frequently. However, as viruses associated with cardiac diseases, various viruses such as influenza virus, Coxsackie A virus, cytomegalovirus, parainfluenza virus are reported in addition to Coxsackie B virus, and the major causative virus has not been ascertained yet (Akira Matsumori, Circular System Now No. 6: Cardiomyopathy/Myocarditis, Nanko-Do Co., p. 36, 1994).
The search for a causative virus has been performed mainly by an indirect method in which a virus antibody is detected in a serum. There has been almost no direct proof from the cardiac muscle. One of the reasons for this is that, since a virus which has invaded the cardiac muscle disappears in several days, it is almost impossible to identify a causative virus in the cardiac muscle, particularly in a clinical scene.
Recently, by the application of dot blotting, slot blotting and nucleic acid hybridization techniques such as in situ hybridization, a study using a cardiac muscle biopsy specimen has become possible. As a result, it has been shown that viral genes remain in tissues for a longer period than expected, and the possibility of viral continuous infection has been suggested. Also, by the recently developed polymerase chain reaction (PCR) method, it has become possible to detect with a higher sensitivity viral genome in cardiac muscle biopsy and autopsy specimens.
However, even with these genetic analysis techniques, results of viral genome detection appear to vary since a region of DNA used as a probe or a region amplified by PCR is different in individual studies. Thus, standardized results have not yet been obtained.
In viral myocarditis, the necrosis and slough of cardiac muscle cells of very wide range and a remarkable cellular infiltration of macrophages and NK cells into the cardiac muscle tissue are observed a at the acute phase. It is possible to consider that cellular immunity is induced by viral infection in cardiac muscles. As a result, those cardiac muscle cells infected with a virus are disrupted by NK cells, etc. at the acute phase. Subsequently, at the subacute phase when most of the virus disappears and thereafter, mainly T cells infiltrate. It is considered that these T cells specifically recognize some antigen expressed on cardiac muscle cells and attack cardiac muscle cells, to thereby cause delayed myocardial disorders.
In this case, it has been also shown that tumor necrosis factor (TNF-.alpha.) acts to worsen myocarditis in an encephalomyocarditis (EMC) viral myocarditis model in mouse (T. Yamada et al., Circulation, 89, 846, 1994). Thus, it is suggested that specific cytokines are also involved in the progress of morbid state.
At present, there is no method established as a specific treatment method for viral myocarditis. However, it has been reported recently that interferon .alpha. (IFN-.alpha.) was effective for a viral myocarditis which appeared to be caused by coxsackievirus B2 and dilated cardiomyopathy-like conditions developed after the myocarditis (A. Heim et al., Clin. Cardiol., 17, 563, 1994). On the other hand, in an experimental mouse model, the effectiveness of ribavirin (an antiviral agent), FK565 (an immune activator), captopril (angiotensin converting enzyme (ACE) inhibitor) and the like has been shown in addition to IFN-.alpha. (A. Matsumori, Viral Infections of the Heart (ed. J. E. Bamatvala), Edward Armold, London, p. 110, 1993). In actual clinical scenes, however, at present symptomatic treatment using a cardiac or the like is conducted while monitoring the cardiac functions, rather than a positive antiviral therapy, since the identification of virus has not been established.
Viral myocarditis is a clinical problem particularly because it is considered that the occurrence of not only myocarditis per se but also delayed myocardial disorders may result in dilated cardiomyopathy (Chuichi Kawai, Journal of Japan Medical Association, Vol. 111, p. 56, 1994).
Viral myocarditis often leaves cardiac abnormalities. In other words, about 43% of viral myocarditis patients who have overcome the acute phase are completely cured, but about 40% of them have sequelae and 3.2% of them suffer from recurrence or exacerbation (Keishiro Kawamura et al., 1982 Study Report by the Survey Group on the Specifically Designated Disease by Ministry of Health and Welfare--Idiopathic Cardiomyopathy, p. 16, 1983). Furthermore, in a subsequent trace survey, it has been made clear that a part of those who had had sequelae presented a dilated cardiomyopathy-like morbid picture. Thus, even after the acute phase, a trace for a long term is considered necessary.
Dilated cardiomyopathy is a disease in which the cardiac muscle undergoes a primary degeneration or functional disorders to thereby develop contraction failure and the ventricles are expanded to thereby present the conditions of congestive heart failure. The prognosis of this disease is quite bad. The survival ratios of 5 years and 10 years after the occurrence are 54% and 36%, respectively (Chuichi Kawai et al., 1982 Study Report by the Survey Group on the Specifically Designated Disease by Ministry of Health and Welfare--Idiopathic Cardiomyopathy, p. 63, 1983). For this reason, dilated cardiomyopathy has become one of the most preferential diseases for the application of heart transplantation in Europe and the United States. In Japan, toward the elucidation of its cause and the establishment of its treatment method, dilated cardiomyopathy has become the specifically designated disease by the Ministry of Health and Welfare and a study group has been organized.
Treatment for dilated cardiomyopathy is mainly symptomatic treatment using a digitalis formulation, diuretic, .beta. blocker, ACE inhibitor or the like. Although these internal treatment may be effective when the reserve force still remains in the cardiac muscle of a patient, the cardiac muscle reaches its limit through repetition of heart failures. Finally, heart transplantation is necessary for such a patient. With respect to hypertrophic cardiomyopathy, a part of this disease is considered due to myosin heavy chain genetic abnormality, but the cause is unknown for the most part of it. Shift from viral infection cannot be ignored as a cause. A method for treating this disease also has not been established yet.
As described above, a treatment method with high specificity has not yet been established for viral myocarditis as well as dilated cardiomyopathy and hypertrophic cardiomyopathy which develop in association with the myocarditis. The development of such a treatment method is urgently required. In particular, dilated cardiomyopathy appears to be an "incurable disease" in the present situation. Thus, it is extremely significant to develop a therapeutic agent and a testing method for this disease.